共查询到20条相似文献,搜索用时 15 毫秒
1.
《International Journal of Cast Metals Research》2013,26(1-4):103-107
AbstractMCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently for conditioning liquid metal prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence on the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys. 相似文献
2.
《International Journal of Cast Metals Research》2013,26(1):65-68
AbstractA novel method for nucleation and detachment of dendrites from a vibrating chilling solid surface inserted into the NH4Cl–H2O alloy melt has been proposed to further increase the proportion of equiaxed grains in the solidification microstructure. The surface nucleation and evolution behaviours of dendrites on the chilling generator with vibration were observed, and the effects of vibration frequency as well as amplitude on the equiaxed crystallographic morphology were also experimentally studied. The results show that exerting vibration to the chilling generator is helpful to not only prevent the formation of solidifying shell but also promote to form equiaxed grains microstructure, and that the higher vibration frequency and amplitude are, the much finer equiaxed grains are obtained. 相似文献
3.
《International Journal of Cast Metals Research》2013,26(1-4):339-342
AbstractCasting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via β-(Ti), e.g. a pathway that avoids peritectic growth of α-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic α (Ti) in the deep mushy zone is significant. 相似文献
4.
《International Journal of Cast Metals Research》2013,26(1-4):226-230
AbstractThe synthesis of Al–3Ti–0˙75C master alloy grain refiner was carried out by adding K2TiF6 and graphite powder together in aluminium melt from 800 to 1200°C. The reaction between the halide salts and graphite in aluminium melt was also carried out at different time intervals at 1200°C. The in situ formation of TiC particles in the Al/K2TiF6/graphite system involved the formation of Al3Ti followed by formation of TiC at varying temperature and time. It is observed that TiC and Al3Ti phases together showed the best grain refining efficiency in comparison to only by TiC or Al3Ti alone. 相似文献
5.
J. Campbell 《International Journal of Cast Metals Research》2013,26(3):156-160
The author shows that the call for high-integrity accurate thin-walled castings in light alloy has resulted in a reappraisal of sand-based moulding methods which might provide designers with an economic answer to their needs. It is suggested that several sand-based processes can now produce results equal or superior to traditional diecasting routes based on gravity or pressure fill. Amongst subjects discussed are low-pressure-filled zircon and silica sand moulds; core-assembly techniques; the use of expendable expanded polystyrene patterns and the V-Process. 相似文献
6.
W. J. Villacis Verdesoto J. A. Sikora 《International Journal of Cast Metals Research》2013,26(4):210-215
The increasing adoption of spheroidal-graphite (s.-g.) cast irons for use in safety critical parts calls for a study of the materials' fracture mechanics. It is becoming necessary to analyse the reliability criteria of these materials on a more rigorous basis. This paper describes studies based upon qualitative and quantitative analysis of crack initiation and propagation under the action of fatigue and three-point bending loads on s.-g. cast irons with the same graphite nodularisation characteristics, but having matrices exhibiting different quantities of ferrite, pearlite and cellular-contour carbides. 相似文献
7.
《International Journal of Cast Metals Research》2013,26(3):181-186
AbstractThe influence of grain refinement and dissolved hydrogen on the fluidity of A356 alloy has been investigated. A spiral casting test method, recently developed, has been used to measure fluidity in a reproducible way. The grain refinement reduces the grain size of the spirals, particularly at the tip, but no significant influence on the fluidity has been revealed. The hydrogen additions in the melt have not affected the fluidity but have, of course, significantly increased the porosity. 相似文献
8.
The mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing were investigated. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl2O4, which displays an equiaxed and faceted morphology with {1 1 1} planes exposed as its natural surfaces. Depending on the Mg content, MgAl2O4 particles exist either as oxide films in dilute Al-Mg alloys (Mg < 1 wt.%) or as naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg > 1 wt.%). Such MgAl2O4 particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl2O4 and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl2O4 particles with a lattice misfit of 1.4% and the increased number density of MgAl2O4 particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl2O4 particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl2O4 particles have been naturally dispersed into individual particles by the increased Mg content. 相似文献
9.
《International Journal of Cast Metals Research》2013,26(1-4):39-44
AbstractA research programme has been undertaken to achieve a more detailed understanding of graphite nucleation control in grey cast irons, at different sulphur (0˙02–0˙1%), residual aluminium (0˙001–0˙010%) and zirconium (0˙001–0˙015%) levels in iron melts. It was found that three groups of elements are important to sustain a three stage model for the nucleation of graphite in grey irons:(i) strong deoxidising elements (Al, Zr) to promote early formed very small microinclusions, oxide based, which will act as nucleation sites for later formed complex (Mn,X)S compounds(ii) Mn and S to sustain MnS type sulphide formation(iii) inoculating elements (Ca, Sr, etc.) which act in the first stage or/and in the second one of graphite formation, to improve the capability of (Mn,X)S compounds to nucleate graphite.It was confirmed that 0˙07%S level is beneficial for graphite nucleation in grey irons with a lower incidence of carbides and undercooled graphite, compared to 0˙023%S cast irons. Low residual Al level (0˙001–0˙003%) results in higher chill and more undercooled graphite and lower eutectic cell count, in inoculated irons. A 0˙007–0˙010%Al content in the melt is important to sustain type A graphite nucleation and reduced chill. Not only inoculation but also the preconditioning (Al or/and Zr) of the base iron has a strong beneficial effect on the solidification pattern of cast irons. Both Al and Zr sustain the type A graphite formation with a lower degree of undercooling and free carbides. These elements were associated in a complex alloy (FeSi based), very efficient in preconditioning of grey irons for thin wall castings, at a low addition rate. 相似文献
10.
11.
《International Journal of Cast Metals Research》2013,26(1-4):135-138
AbstractUltrasound is defined in terms of human hearing and it is a sound having a frequency higher than that to which the human ear can respond. The attempts to use the ultrasound at casting process have been carried out and there are a lot of study results on grain refinement on microstructure of casting alloys. However, these studies almost have been focused on mould vibration at solidifying melts of solid and liquid coexistence temperature. In this study, high intensity ultrasound was injected in aluminium full melts, especially A390 alloy, to evaluate the effect of ultrasound on microstructure and mechanical properties of castings. The effect of ultrasonic injection on melts could be summarised as follows: reduction of mean size of primary silicon, variation of phase distribution, improvement of solute homogeneity and mechanical properties. 相似文献
12.
Yinon Ashkenazy Nhon Q. VoDaniel Schwen Robert S. AverbackPascal Bellon 《Acta Materialia》2012,60(3):984-993
Shear-induced mixing in heterogeneous Cu alloy systems was investigated by molecular dynamics simulation. Each system, which comprised a single spherical particle within a Cu matrix, was subjected to cyclical shearing events to high strains at 100 K. The particles investigated were Cu, Ag, Ni, Fe, Nb, and V. fcc particles were observed to undergo “superdiffusive” mixing with dislocations crossing the particle-matrix interface. The initial rate of mixing in these systems increased quadratically with particle radius. bcc particles showed different behaviors. For Nb and V dislocations did not cross into the particles and the initial rates of mixing increased linearly with particle radius. The Nb particles remained spherical but developed amorphous Cu-Nb shells at the particle-matrix interface. The V particle showed some faceting, but it too formed amorphous layers on non-facetted interfaces. The Fe particle behaved similarly to Nb and V particles initially, but more like fcc particles at high strains. The molecular dynamics results are compared with experimental observations. 相似文献
13.
《Acta Materialia》2007,55(2):565-574
The phase field model is employed to study the structural mechanism of heterogeneous nucleation in the face-centered cubic to body-centered cubic martensitic transformation. The transformation is triggered in the undercooled parent phase by defects of varying potency, a dislocation loop or a group of loops. It is found that the metastable homogeneous state of the parent phase in this case is not necessarily stable even if the undercooling is small and the potency of defects is low – the parent phase transforms around the defects, forming a sessile metastable martensitic embryo. When the undercooling and the defect potency reach a critical value, the embryo loses its metastability with respect to the barrierless growth until the transformation is complete. This growth corresponds to an athermal martensitic transformation. It is shown that both sessile and growing martensitic embryos are not a single-domain particle as is usually assumed but rather an assemblage of twin-related domains. The modeling does not impose any a priori constraint on the possible microstructure of a martensitic nucleus. 相似文献
14.
Intensive melt shearing was applied to the commercial AZ91D alloy melt to investigate its effects on grain refinement. Alloy melts with and without melt shearing were also filtered using a pressurized filtration technique to concentrate the potential nucleating particles for electron microscopic examination. The results showed that intensive melt shearing resulted in significant refinement of both the Al8Mn5 intermetallics and the primary α-Mg phase in the as-cast AZ91D alloy, and that this grain-refining effect is insensitive to the superheat and can persist even after prolonged isothermal holding. The pressurized filtration experiments showed for the first time that oxide films and skins consist of nano-sized MgO particles populated densely in a liquid matrix. Intensive melt shearing can effectively disperse such MgO particles throughout the alloy melt. The HRTEM investigation and detailed crystallographic analysis confirmed that dispersed MgO particles act as potent heterogeneous nucleation sites for both the Al8Mn5 and α-Mg phase. 相似文献
15.
通过弹道冲击试验和高温Hopkinson Bar压缩试验研究了Mg-Gd-Y系合金的绝热剪切行为.利用光学显微镜对试验后试样的组织进行观察.结果发现,该合金在两种试验条件下形成两种绝热剪切带.弹道冲击过程中形成的剪切带在光学显微镜下呈白亮色,平均宽度约为10 μm,属于白亮带,带内组织的显微硬度明显高于周围基体.白亮带只能在稳定塑性侵彻阶段形成:整个高温Hopkinson Bar压缩过程中只在T=735 K时形成较明显的塑性变形带.远离绝热剪切带源点的裂纹的形成主要是由平行于剪切方向的孪晶引起的. 相似文献
16.
Hot shear spinning experiments with Mg–3.0 Al–1.0 Zn–0.5 Mn(AZ31 B, wt%) magnesium alloy sheets were conducted at various temperatures, spindle speeds and feed ratios to investigate the effects of these processing parameters on the microstructure, crystallographic texture and mechanical properties. The AZ31 B sheet displayed good shear formability at temperatures from 473 to 673 K, spindle speeds from 300 to 600 rev/min and feed ratios from 0.1 to 0.5 mm/rev. During the dynamic recrystallization process, the grain size and texture were affected by the deformation temperature of the hot shear spinning process. Each of the spun sheets presented a strong basal texture, and the c-axis of most of the grains was parallel to the normal direction. The optimal hot shear spinning parameters were determined to be a temperature of 473 K, a spindle speed of 300 rev/min and a feed ratio of 0.1 mm/rev. The yield strength, ultimate tensile strength and elongation in the rolled direction reached 221 MPa, 288 MPa and 14.1%, and those in the transverse direction reached 205 MPa, 280 MPa and 12.4%, respectively. The improved strength and decreased mechanical anisotropy resulted from the fine grain size and strong basal texture. 相似文献
17.
针对钢中添加稀土镧、铈形成的高熔点夹杂物相Ce_2O_3和La_2O_3,利用边-边匹配模型计算其与初生相(δ-Fe或γ-Fe)之间的原子匹配情况,从晶体学角度预测其作为钢液凝固时初生相异质形核核心的可能性及效用,并为理解稀土细化钢的凝固组织的作用机理提供新的见解。结果表明,Ce_2O_3和La_2O_3均可作为初生相δ-Fe及γ-Fe异质形核的核心;Ce_2O_3和La_2O_3均可有效细化δ-Fe晶粒,但其细化γ-Fe晶粒的效果远不如其细化δ-Fe晶粒的效果;对于γ-Fe,La_2O_3的细化效果较Ce_2O_3稍好。 相似文献
18.
《International Journal of Cast Metals Research》2013,26(1-4):23-26
AbstractThe form and distribution of graphite in grey iron influences the mechanical properties and depends on numerous factors, such as nucleation and cooling speed conditions. The main focus of the present work is the influence of manganese, sulphur and oxygen on the nucleation of graphite in lamellar cast iron melts. Previous studies showed that the nucleation in GJL melts is initiated by a MnS particle. For investigations in the field of nucleation in grey iron the authors examined several EN-GJL-200 specimens. The specimens were cast with and without inoculant. The studies of the specimens were realised using light microscopy, SEM-EDS (energy dispersive X-ray analysis), WDS (wavelength dispersive X-ray analysis). The experimental results were compared with the software Thermo-Calc calculations. From the experimental results and the Thermo-Calc calculations it can be concluded, that Mn and S contents and the Mn/S ratio respectively plays a very important role for the nucleation. A direct influence of oxygen on the nucleation of graphite could not be observed. 相似文献
19.
铝及其合金中溶气的物理化学 总被引:2,自引:0,他引:2
应用化学热力学及动力学的基本原理,在阐明铝合金中气体反应行为基础上,根据铝合金的吸氢及析氢机理.从理论上分析了铝合金中针孔成因,进而得出了针孔严重程度的定量判据式(即计算模型)。 相似文献
20.
《International Journal of Cast Metals Research》2013,26(1-4):172-174
AbstractThis paper deals with the validation of a multiphase solidification model based on a benchmark experiment presented in Part 1. For the numerical modelling of NH4Cl–H2O solidification, the three different phases liquid, columnar dendrite trunks and equiaxed grains have been considered. The mass, momentum, energy conservation and species transport equations for each phase have been solved. The multiphase Eulerian-Eulerian model equations have been implemented in the Finite Volume Method based commercial software FLUENT-ANSYS using User-Defined Functions (UDF). The simulation of the NH4Cl–H2O solidification has been numerically investigated as a twodimensional unsteady process representing a cross-section of a 100 × 80 × 10 mm experimental benchmark. During the experiment both columnar and equiaxed growth of NH4Cl were observed, therefore both phenomena were considered in the simulation. The predicted distribution of the solidification front has been compared with the measurements. 相似文献